Stratigraphy is the study of rock and soil layers to establish relative dating. The basic principle is simple: in an undisturbed sequence, lower layers are older than the layers above them. Archaeologists use this to build a sequence of occupation at a site. If a pottery style appears in a lower layer than another style, the first is older. That reasoning, called the law of superposition, was articulated by Nicolas Steno in the 17th century.

Real sites are rarely undisturbed. Rodents burrow. Roots push through. Floods erode. Later pits cut into earlier layers. Archaeologists record the exact position of every find and draw section diagrams showing the layers in profile. They also watch for intrusions, objects that have moved from their original context. A coin found in a deep layer is suspicious until proven otherwise. Cross-dating with other sites and absolute methods like radiocarbon helps confirm sequences.

Key principles

• Superposition: lower layers are older.
• Original horizontality: layers start out flat.
• Cross-cutting: a feature that cuts another is later.
• Association: objects in the same layer are roughly contemporary.

Stratigraphy gives archaeology its time depth. Without it, artifacts would float without context.